This invention is directed to a logging-while-drilling (LWD) and measurement-while-drilling (MWD) approach for obtaining nuclear magnetic resonance (NMR) data concerning petrophysical properties of a formation. More specifically, the invention is directed to an improved accuracy method and device for reducing the sensitivity of NMR measurements to tool motions, and real-time transmission over a slow data channel, such as mud pulsing for quick-look results at the surface.
LWD and MWD systems are generally known in the art to make downhole measurements while a borehole is being drilled. Such systems measure various parameters and characteristics of the formation, such as its resistivity and the natural gamma ray emissions from the formation. Typically, signals which are representative of these measurements made downhole are relayed to the surface with a mud pulse telemetry device that controls the mud flow, encoding information in pressure pulses inside the drill string. The pulses travel upward through the mud to the surface, where they are detected and decoded so that the downhole measurements are available for observation and interpretation at the surface substantially in real time. In addition, it has also been found useful to provide a downhole computer with sufficient memory for temporarily storing these measurements until such time that the drill string is removed from the borehole.
U.S. Pat. No. 5,280,243 to Miller discloses an NMR apparatus and method of use for geophysical examination of a bore hole as it is being drilled. The patented apparatus is connected to the drill bit and follows it through the bore hole as it is being formed. In operation, the apparatus generates a gradient static magnetic field in a region of the bore hole adjacent the apparatus. This static field extends radially with respect to the longitudinal axis of the apparatus and has a generally uniform amplitude along the azimuth with respect to that axis. Next, a pulsed radio frequency magnetic field is generated to excite nuclei in a substantially cylindrical shell around the tool that defines in the formation a sensitive region extending along the length of the tool and having thickness of about 1 mm. Due to this relatively narrow sensitive region, standard wireline NMR relaxation time measurements are difficult to perform with this tool because lateral vibrations during the measurement time would reduce the accuracy of the measurement.
U.S. Pat. 5,557,201 to Kleinberg et al. discloses a pulsed NMR device in which the accuracy of the measurement with respect to lateral tool vibrations is enhanced by providing a larger sensitive region. This is achieved by a special tool architecture shown in FIGS. 2A-B, using two tubular permanent magnets 22 with same poles facing each other, and an antenna 26 positioned in the recess between the two magnets. In operation, this tool architecture provides a sensitive region in the formation which is larger laterally, but is greatly reduced along the borehole axis, because of the presence of a single stationary point in the formation. It is expected therefore that vertical tool motions would affect the accuracy of the tool measurements.
Current NMR LWD and MWD applications also suffer from the drawback that tool operators have no way of determining whether a portion of the formation which is being drilled at a given time is of interest or not. Typically, during the drilling process the measurement tool rotates along with the drill bit and is used in the more robust T1 measurement mode. On the other hand, due to its better accuracy T2 measurement mode is preferred for investigations of formation zones that are considered to be of higher interest. Such measurements are conducted while the drilling has been stopped, because the vibrations of the entire assembly during drilling interfere with the accuracy of the T2 measurements. Thus, if the operator wishes to revisit interesting zones traversed by the borehole in the formation he has to compile a log of such zones in repeat the logging process separately.
Accordingly, it is perceived that there is a need for a system and method with improved sensitivity with respect to tool motions of pulsed NMR measurements using pulsed NMR tools. It is also perceived that there is a need for a system and method capable of providing real-time data concerning the properties of zones within the formation which are being investigated using LWD and MWD techniques.
The present invention concerns a novel method and device for formation evaluation while drilling a borehole using pulsed NMR tools with magnetic fields that are rotationally symmetric about the longitudinal axis of the borehole.
In a preferred embodiment, the method of the present invention is based on NMR relaxation time measurements determining longitudinal relaxation times T1. In particular, the method comprises the steps of generating at least one radio frequency pulse covering a relatively wide range of frequencies to saturate the nuclear magnetization in a cylindrical volume around the tool; transmitting a readout pulse at a frequency near the center of the range of covered frequencies, the readout pulse following a predetermined wait time; applying at least one refocusing pulse following the readout pulse; receiving at least one NMR echo corresponding to the readout pulse; repeating the above steps for a different wait time to produce a plurality of data points on a T1 relaxation curve; and processing the produced T1 relaxation curve to derive petrophysical properties of the formation.
In another aspect, the invention is a method for making nuclear magnetic resonance (NMR) measurements of a geologic formation using a NMR logging tool, comprising the steps of: providing a static magnetic field in a volume of said formation; applying oscillating magnetic fields according to a pulse sequence
xcfx84ixe2x88x92xcfx80/2(+x)xe2x88x92[tcpxe2x88x92xcfx80xe2x88x92tcpxe2x88x92echo]jxe2x88x92tcpxe2x88x92xcfx80/2(xe2x88x92x) 
where xcfx84i is a variable delay, and ixe2x89xa71; jxe2x89xa71; and +x and xe2x88x92x denote phases of the Larmor frequency of the carrier of the pulse with respect to a continuous wave Larmor frequency signal; tcp is the Carr-Purcell spacing; and measuring the induced NMR echo signals. an optimized pulse sequence for use in T1 logging.
In yet another aspect, the invention is a method for real-time processing of downhole logging data, comprising the steps of: a) drilling a borehole into a geologic formation; b) while drilling the borehole, applying a first data acquisition sequence to determine substantially in real time at least one parameter of a zone in the formation being traversed; c) selecting a second data acquisition sequence based upon the at least one determined parameter; and e) applying the selected second data acquisition sequence to determine additional properties of said zone of the formation.
In another aspect, the invention is a method for making nuclear magnetic resonance (NMR) measurements of a geologic formation using an NMR logging tool, comprising the steps of: applying oscillating magnetic fields according to a first pulse sequence
xcfx84ixe2x88x92xcfx80/2(+x)xe2x88x92[tcpxe2x88x92xcfx80xe2x88x92tcpxe2x88x92echo]jxe2x88x92tcpxe2x88x92xcfx80/2(xe2x88x92x) 
where xcfx84i is a variable delay, and ixe2x89xa71; jxe2x89xa71; and +x and xe2x88x92x denote phases of the Larmor frequency of the carrier of the pulse with respect to a continuous wave Larmor frequency signal; tcp is the Carr-Purcell spacing; applying one or more times a chirped pulse sequence, comprising a radio frequency (RF) pulse covering a relatively wide range of frequencies to saturate nuclear magnetization in a volume within the geologic formation and a readout pulse sequence at a frequency within the range of covered frequencies, the readout pulse sequence following a predetermined wait time after the saturation pulse; receiving NMR echo signals corresponding to the first pulse sequence and to the one or more chirped pulse sequence; and processing the received NMR echo signals to determine properties of the geologic formation.
Additional aspect of the invention are described in further detail below.